Rotary steerable drilling systems for drilling deviated boreholes into the earth are generally classified either as point-the-bit systems or push-the-bit systems. In point-the-bit systems, the axis of rotation of the drill bit is deviated from the local axis of the bottom hole assembly in the general direction of the new portion of the hole being drilled. The borehole is propagated according to customary three-point geometry defined by upper and lower stabilizer touch points and the drill bit. The angle of deviation of the drill bit axis coupled with a finite distance between the drill bit and a lower stabilizer results in a non-collinear condition required for a curve to be generated. In this type of system, the drill bit tends to have less sideways cutting because the bit axis is continually rotated in the direction of the curved borehole.
In push-the-bit rotary steerable systems, there is usually no specially identified mechanism to deviate the bit axis from the local bottom hole assembly axis. Instead, the requisite non-collinear condition is achieved when either upper or lower stabilizers are used to apply an eccentric force or displacement in a direction oriented with respect to the direction of borehole propagation. Steering is again achieved by creating non co-linearity between the drill bit and at least two other touch points. In this type of system, the drill bit is required to cut sideways to generate the desired, curved borehole.
In many of these rotary steerable systems, pistons may be used to create force against a borehole wall or to cause angular displacement of one steerable system component with respect to another to cause the drill bit to move in the desired direction of deviation. The pistons are deployed in a piston actuated mechanism and forced to their desired displacement to achieve the directional control. The pistons are manipulated via drilling mud pumped down through the bottom hole assembly. However, such systems may be subjected to internal wear from the flowing mud and also may be limited with respect to the forces which may be applied to steer the drill bit.